This invention generally relates to a light emitting device, and more specifically to an organic electroluminescent device containing a light emitting material having an indole skeleton and good light-emitting properties.
Organic electroluminescent devices (organic EL devices) arc light emitting devices containing a fluorescent material which emits light in response to the recombination of holes and electrons injected from the anode and cathode (C. W. Tang et al. Applied Physics Letters, 1987, 51:913). The luminescence efficiency of organic EL devices can be improved by doping with a fluorescent dye. For instance, doping with a coumarin dye can greatly improve the luminescence efficiency of an organic EL device (Applied Physics letters, 1989, 65:3610). A well-known coumarin dye is C-545T (U.S. Pat. No. 4,769,292), which has the following structure: 
For improving the recombination efficiency of the injected holes and electrons, multi-layered devices have been introduced. A hole transporting layer (HTL) containing a hole transporting material (HTM) is used to improve hole injections and transporting from the anode into an organic layer. An example of a well-known HTM is NPB (4,4xe2x80x2-bis [N-(1-naphthyl)-N-phenyl-amino-]bisphenyl), which has the following structure: 
An electron transporting layer (ETL) containing an electron transporting material (ETM) is used to improve the electron injection from the cathode into the organic layer. A typical ETM, Alq3 (aluminum tris (8-hydroxyquinolate)), has the following structure: 
Other materials, such as oxadiazole compounds, triazine compounds, and triazole compounds, can also be used as ETMs.
Aromatic dimethylidyne compounds have been used as blue light emissive materials for organic EL devices (U.S. Pat. No. 6,093,864). An example of aromatic dimethylidyne compounds is (1,4-bis 2,2-di-phenylvinyl)biphenyl (DPVBi) having an EL peak at about 485 nm and the following structure: 
One of the objects of the present invention is to provide a new light emitting material based on an indole skeleton and another object is to provide an organic EL device which has good light emitting properties containing the new light emitting material. The organic EL device comprises an anode, cathode, and one or more organic thin film layers which contain one or more indole-based compounds represented by Formula (1): 
wherein Ar1 represents a substituted or unsubstituted aromatic hydrocarbon group, a group formed by two aromatic hydrocarbon groups directly linked together, wherein the two aromatic hydrocarbon groups are independently substituted or unsubstituted, a substituted or unsubstituted aromatic heterocyclic group, a group formed by two aromatic heterocyclic groups directly linked together, wherein the two aromatic heterocyclic groups are independently substituted or unsubstituted, or a group formed by an optionally substituted aromatic hydrocarbon group directly linked with an optionally substituted aromatic heterocyclic group; R1, R2 and R3 each independently represents a H atom, F atom, CN group, substituted or unsubstituted alkyl group, substituted or unsubstituted aromatic hydrocarbon group, or substituted or unsubstituted amino group; any two of R1, R2 and R3 together with the attached carbon atoms may form an aromatic heterocyclic or hydrocarbon ring; and R4 represents a substituted or unsubstituted alkyl group, substituted or unsubstituted aromatic hydrocarbon group, or substituted or unsubstituted aromatic heterocyclic group. Within the scope of the invention are the indole-based compounds represented by Formula (1).